Note: Automatically acquired by ALTER TABLE,
DROP TABLE, VACUUM statements.
This is the most restrictive lock mode which conflicts with all other
lock modes and protects a locked table from any concurrent operations.

Note: This lock mode is also acquired by an unqualified
LOCK TABLE (i.e. the command without an explicit
lock mode option).

Outputs

LOCK TABLE

The lock was successfully applied.

ERROR name: Table does not exist.

Message returned if name
does not exist.

Description

LOCK TABLE controls concurrent access to a table
for the duration of a transaction.
Postgres always uses the least restrictive
lock mode whenever possible. LOCK TABLE
provided for cases when you might need more restrictive locking.

RDBMS locking uses the following terminology:

EXCLUSIVE

Exclusive lock that prevents other locks from being granted.

SHARE

Allows others to share lock. Prevents EXCLUSIVE locks.

ACCESS

Locks table schema.

ROW

Locks individual rows.

Note: If EXCLUSIVE or SHARE are not specified, EXCLUSIVE is assumed.
Locks exist for the duration of the transaction.

For example, an application runs a transaction at READ COMMITTED isolation
level and needs to ensure the existance of data in a table for the
duration of the
transaction. To achieve this you could use SHARE lock mode over the
table before querying. This will protect data from concurrent changes
and provide any further read operations over the table with data in their
actual current state, because SHARE lock mode conflicts with any ROW EXCLUSIVE
one acquired by writers, and your
LOCK TABLE name IN SHARE MODE
statement will wait until any concurrent write operations commit or rollback.

Note: To read data in their real current state when running a transaction
at the SERIALIZABLE isolation level you have to execute a LOCK TABLE
statement before execution any DML statement, when the transaction defines
what concurrent changes will be visible to itself.

In addition to the requirements above, if a transaction is going to
change data in a table then SHARE ROW EXCLUSIVE lock mode should
be acquired to prevent deadlock conditions when two concurrent
transactions attempt to lock the table in SHARE mode and then
try to change data in this table, both (implicitly) acquiring
ROW EXCLUSIVE lock mode that conflicts with concurrent SHARE lock.

To continue with the deadlock (when two transaction wait one another)
issue raised above, you should follow two general rules to prevent
deadlock conditions:

Transactions have to acquire locks on the same objects in the same order.

For example, if one application updates row R1 and than updates
row R2 (in the same transaction) then the second application shouldn't
update row R2 if it's going to update row R1 later (in a single transaction).
Instead, it should update rows R1 and R2 in the same order as the first
application.

Transactions should acquire two conflicting lock modes only if
one of them is self-conflicting (i.e. may be held by one
transaction at time only). If multiple lock modes are involved,
then transactions should always acquire the most restrictive mode first.

An example for this rule was given previously when discussing the
use of SHARE ROW EXCLUSIVE mode rather than SHARE mode.

Note: Postgres does detect deadlocks and will
rollback at least one waiting transaction to resolve the deadlock.

Notes

LOCK is a Postgres
language extension.

Except for ACCESS SHARE/EXCLUSIVE lock modes, all other
Postgres lock modes and the
LOCK TABLE syntax are compatible with those
present in Oracle.

LOCK works only inside transactions.

Usage

Illustrate a SHARE lock on a primary key table when going to perform
inserts into a foreign key table:

BEGIN WORK;
LOCK TABLE films IN SHARE MODE;
SELECT id FROM films
WHERE name = 'Star Wars: Episode I - The Phantom Menace';
-- Do ROLLBACK if record was not returned
INSERT INTO films_user_comments VALUES
(_id_, 'GREAT! I was waiting for it for so long!');
COMMIT WORK;

Take a SHARE ROW EXCLUSIVE lock on a primary key table when going to perform
a delete operation: